Approaches have been developed to allow multiple users to reuse a single timeslot in wireless systems, referred to as Multiple Users Reusing One Slot (MUROS) technologies. One such approach involves the use of orthogonal sub-channels (OSC). The OSC concept allows a wireless network to multiplex two wireless transmit/receive units (WTRUs) that are allocated the same radio resource (i.e., time slot). In the uplink direction, the sub-channels are separated by using non-correlated training sequences. The first sub-channel can use existing training sequences, and the second sub-channel can use new training sequences. Alternatively, only new training sequences may be used on the sub-channels. Using OSC can enhance voice capacity with negligible impact to WTRUs and networks. OSC can be transparently applied for all Gaussian minimum shift keying (GMSK) modulated traffic channels (e.g., for full rate traffic channels (TCH/F), half rate traffic channels (TCH/H), a related slow associated control channel (SACCH), and a fast associated control channel (FACCH)).
OSC increases voice capacity by allocating two circuit switched voice channels (i.e., two separate calls) to the same radio resource. By changing the modulation of the signal from GMSK to QPSK (where one modulated symbol represents two bits), it is relatively easy to separate two users—one user on the X axis of the QPSK constellation and a second user on the Y axis of the QPSK constellation. A single signal is used but it contains information for two different users, each user allocated their own sub-channel.
In the downlink, OSC is realized in the transmitter of a base station (BS) using a quadrature phase shift keying (QPSK) constellation that may be, for example, a subset of an 8-PSK constellation used for enhanced general packet radio service (EGPRS). Modulated bits are mapped to QPSK symbols (“dibits”) so that the first sub-channel (OSC-0) is mapped to the most significant bit (MSB) and the second sub-channel (OSC-1) is mapped to the least significant bit (LSB). Both sub-channels may use individual ciphering algorithms, e.g., A5/1, A5/2 or A5/3. Several options for symbol rotation may be considered and optimized by different criteria. For instance, a symbol rotation of 3π/8 would correspond to EGPRS, a symbol rotation of π/4 would correspond to π/4-QPSK, and a symbol rotation of π/2 can provide sub-channels to imitate GMSK. Alternatively, the QPSK signal constellation can be designed so that it appears like a legacy GMSK modulated symbol sequence on at least one sub-channel.
An alternate approach to realizing the MUROS concept in the downlink involves multiplexing two WTRUs together by transmitting two individual GMSK-modulated bursts per timeslot. As this approach causes increased levels of inter-symbol interference (ISI), an interference-cancelling technology such as DARP Phase I or Phase II should be employed in the receivers. Typically, during the OSC mode of operation, the BS applies downlink and uplink power control with a dynamic channel allocation (DCA) scheme to keep the difference of received downlink and/or uplink signal levels of co-assigned sub-channels within, e.g., a ±10 dB window, although the targeted value may depend on the type of receivers multiplexed together and other criteria. In the uplink, each WTRU can use a normal GMSK transmitter with an appropriate training sequence. The BS typically employs interference cancellation or joint detection type of receivers, such as a space time interference rejection combining (STIRC) receiver or a successive interference cancellation (SIC) receiver, to receive the orthogonal sub-channels used by different WTRUs.
OSC may or may not be used in conjunction with frequency-hopping or user diversity schemes, either in the downlink (DL), in the uplink (UL), or both. For example, on a per-frame basis, the sub-channels may be allocated to different pairings of users, and pairings on a per-timeslot basis may recur in patterns over prolonged period of times, such as several frame periods or block periods.
The OSC concept relies on the sub-channels remaining relatively orthogonal in the DL. When a transmission (Tx) pulse format is used that is spectrally wider than the legacy GMSK pulse format, orthogonality between two OSC sub-channels multiplexed onto the same burst is greatly improved. This also leads to improvements in link performance and receiver design. The improvement in orthogonality can be attributed to a number of causes, including reductions in time dispersion and artificial receiver distortion effects. If orthogonality between sub-channels is not properly maintained, the OSC sub-channels will greatly interfere with each other, even in absence of any actual channel impairment, and will degrade receiver performance for the multiplexed WTRUs. For example, if the legacy relatively narrow GSMK Tx pulse format is used, the pulse shaping filter will introduce ISI and perturb the orthogonality of the sub-channels in the modulated carrier. The introduced ISI may exceed acceptable levels even in static channel environments, producing a block error rate (BLER) of greater than ten percent.
Therefore, it has been proposed that pulse formats wider than the legacy GMSK pulse format be employed. One suggested format is the 270 kHz Hanning windowed Root-Raised Cosine (RRC) pulse, though it exceeds the current GSM/EDGE spectrum mask. However, a disadvantage of a spectrally wider Tx pulse is increased interference levels on channels adjacent to channels where OSC is used. The increase in interference levels may depend on UL and DL characteristics, as well as whether and to what extent frequency-hopping is employed.
Additionally, interference cancellation technology may be employed to combat increased levels of ISI. Noise cancellation technologies that could be employed include techniques such as Downlink Advanced Receiver Performance (DARP) Phase I, whereby one antenna is used and the stronger co-channel interferer on a burst is canceled out, and DARP Phase II, whereby two antennas are used.
While a wider pulse format is more robust than a narrow pulse format and offers distinct advantages when employed in the context of multiplexed or MUROS transmissions, a wider pulse format may have the undesirable side effect of affecting adjacent channels, due to increasing leakage of power (“interference”) into the adjacent channels. This problem is of especial concern for legacy equipment currently in use, as it cannot be redesigned to take this into account for changed interference levels. Even with newly designed equipment, the signal-to-interference ratio (SIR) experienced in adjacent channels may be so severe as to negate possible gains in throughput offered by a wider pulse format. As a function of receiver design, legacy equipment may or may not be able to tolerate various interference levels resulting from the use of wider pulse formats. Further, inter-channel interference may occur when wider pulse formats are used in channels adjacent to channels in another operator's network. Under such a circumstance, special care must be taken when allowing WTRUs to use wider pulse formats, because different network operators do not customarily coordinate the configurations of their respective networks.
Therefore, an approach is required to allow for networks to communicate pulse format information to WTRUs, such that pulse formats on both the UL and DL may be dynamically configured or switched during operation. An approach is also required to allow for the use of a wider pulse format in MUROS transmissions while at the same time taking avoiding increased interference levels that may be caused due to constraints imposed by legacy equipment.